Species and habitat are factors that play into a starfish's longevity.  Starfish in aquariums on average live 8 years but those in the wild can live between 5 and 20 years.  For more information on starfish of all kinds check out the links below.  

This depends on which of over 1800 species of starfish you're referring to. Smaller ones, like Leptasterias hexactis, live for 10 years with maturity at 2. A large species like the Purple Sea Star (Pisaster ochraceus) can live as long as 35 years in the wild.